Turn your head into something useful

http://www.saltmine.org.uk/shoeman/heads.htm

Thanks for this Jimmie,
Great for all beginners! please everyone... read this!
Alan

Compression ratio was mentioned but not the limiting factor of Compression Ratio for petrol fuels under 100 octane. Around 8:1 is safe when using 95octane petrol and at 9:1 its better to get a fuel with a guaranteed over-100 octane. Going high on compression ratio with under 100 octane is dangerous. Australians Walsh & McPhee were obviously running Bantams on alcohol fuel -- methane with an additive most likely. The combustion of tetra-ethyl (lead) fuels (Dope fuels) have lower peak combustion-pressures (than petrols) with longer heat release rates which means compression ratios of 16:1 and above can be used as long as the mixture is always rich.

My mention earlier of a Hot-Plug engine is where high compression ratio with a platinum hot-plug are MUSTS... With this arrangement carburration over a wide operating speed range is difficult for smooth running and the engine usually only runs smooth at high revs.
I´d imagine the Australian engines using this arrangement -- or just high CR and
methane -- would not pass the modern noise test with their imeagphone or open exhausts....

Last point is that from 14:1 to 20:1 CR the Thermal Efficiency is 65% to 70% and at 8:1 is 55% -- that´s when combustion is complete, of course.

Another interesting site which son Mark turned up is at:

http://lambretta-images.com/archive/porttiming.php

Mr. Bass,
You are somewhat behind the times sir! In Bantam terms anyway...

So that Bantam beginners are not left behind any more than necessary, we know that 12:1 with modern pump petrols is reliable on a good 175 Bantam, you obviously need to watch for weak mix and excessive ignition timing, but it really is reliable!
For a straigght line ignition on pump petrol I would be uncomfortable with greater than 12:5 to 1, but I know some people do it, regardless, and appear to be ok so the choice is yours!

I use Avgas (it is very stable and no tank leak problems!), and I have various compression ratio's available dependent on the circuit, eg: low ish for a long circuit (12.5:1) and a tight ciruit I use all the way up to 15:1. There is always a trade off. and you need to be capable of altering the ignition curve to avoid a serious mechanical explosion! Hence I use a programmable curve.
Note that the higher the compression the more likely your powerband will be trunkated at the top end of the range, this is the trade off you have to make, so you will need to work out a balance.
Hope this is useful
Cheers,
Alan

Of course Alan, you are right, the reason I write garbage on here is to be corrected so that I can catch up with your modern times but I think you were saying virtually the same as I except for the point of Long straights like that of the OLD Norwich Straight at Snetterton wich meant a long period of full throttle which with high CR and petrol can be tempting disaster. .

Hi Alan,
That `s an interesting post , would i be correct in assuming that your advice on possible c/r is applicable to both iron and alloy engines ?
As a point of interest , i never vary the c/r on the w/c engine that Mark campaigns , the thinking being that we have an assured level
of stability in settings and therefore an agreed level of performance at any circuit , seems to work for us !
What i can`t seem to get a handle on is the suggestion that a high c/r truncates the power band at the top end , perhaps this is where
i`ve been going wrong all these years . Maybe you could explain the science , or even the thinking behind this phenomenon of the
combustion process , i`m sorely in need of guidence on this one ! Does the pipe and/or the ignition timing have any influence here ?
Would it also be reasonable to conclude that you program your ignition against the varying points of maximum brake torque , and , if
so, how do you determine that point and also avoid running into potential detonation problems associated with running at that point of
maximum heat production ?

Hope you can help me here , Trevor

Jimmie pointing out this web page has idetified a number of aspects we were not aware of, we have never looked into the effects the conrod length has on timing in degrees advance/retard, but as we run a longer rod than most for a 125 short stroke motor (125mm), 1.2mm seems we are still running timing at 15.5 degress. after we dropped this from 1.8mm it was running on a lot more and really reving, compared to previouse timings /setup.

very helpfull information this.

Alan my compression volume is a shade under 14cc measured it in-situ, we went this large trying to reduce the over heating issues we have been experiencing so my question know is what would you modify first, I would like to put it back to where it was before we started detuning it in order to control the overheating problems we have had

our over heating issue- to recap on this - it was identified - as a result of both the perforated tail pipe and / silencer being too small, we reduced the comp ratio trying to resolve this to 10. to 1.

so what would you suggest we try next
-
? up the comp ratio back to 12.5 to 1 approx 9.8cc -we are running avgas and it is very cool compared to where it has been, and another reason we have had issue with sorting the jetting .

also Peter Tibbetts given us some support on a post on profiling the piston skirt, I have to say its a little simpler to follow, took a little longer but more methodical way - also we have experienced no scuffing and with a new piston and bore size, after it needed boring, We will not know what other improvements this has made untill we get it back on a dyno and put it back to how it was.
[list][*]

point made ????? thanks Derek no probs. Looking forward to doing the ignition....Al

We all know that when the engine is run weak and just beyond its too weak a chemically-correct-mixture-for-complete-combustion*** level it gives a really good performance but I wonder how many know that the aircraft fuel (which Alan mentioned and I am not allowed to [MP announced earlier...]) increases its octane rating from just over 100 to 130 when it it is richened from *** that stated above... I read recently that this function has been used with computerised control of fuel injection where the mixture is weaked off to increase combustion chamber temperature and then richened to prevent overheating damage or seizure which causes an increase of power -- albeit short lived... "Boost of power" might be the better term because it does apply to piston-powered aircraft engines as well.

... or is this ancient history again?

This using microprcessor control could be used with a variable compressio-ratio piston -- as was experimented with by Continental Engines... This is where the increasing piston crown temperature operates an oil pressure system that had -- initially -- kept the two-part piston at High compression ratio thus with reduced oil pressure the piston collapses to a lower CR and overheat damage is avoided....

OK, OK! I got it!! -- enough is enough -- I´m off....

HI Trevor,
I can only speak of my own experience, and experiments (all done on a dyno) and all done on an "all" alloy barrel.
My cylinder head is inserted and hence I have a very easy time experimenting with compression ratio and head styles, as the inserts are easy to make on a small lathe, I have no experience of doing this on an iron barrel.

I am sure that you have not been "going wrong" in any way at all! (results say otherwise) I can only speak in terms of what I see on the dyno graphs that I have produced on my engine. Also the great differences between engines in that you have a "super revver" engine, and I have deliberately avoided this in my motor which only just reachs 10K rpm on a good day, my original reasoning being reliability of the crank.
I believe that as the compression ratio gets higher there is a point where the compressing gas (before the ignition) works against the flywheel effect and tends to reduce the "rev on" of the motor, this is how I explain it, but it is observed by me as an effect.
Certainly within the powerband I use, maybe not so in the powerband you use. But the effect is seen on my motor, note: I have taken a great deal of trouble to correct carburation to make sure that this was not an issue to create confusion.
Your assumption of the digital ignition points is correct, and also tested against the dyno, and I have to say, experience of my ignition systems.
I have had 14 different pipes on my engine so far, and there are more to come! and I have yet to note any real difference, with the engine revving on differently with each pipe there appears to be some element of truncation when I increase the compression ratio. The pipe was chosen for the best overall effect, rather than best horse power figure.
Hope this is reasonable.
Cheers,
Alan

Hello Alan,
Gratefull thanks for all of that infomation , a lot to digest, has me thinking some what !
Just one quick question , for now , has your engine ever reved significantly above 10,000 rpm
or do you deliberately suppress the rev ceiling irrespective of what the dyno may show
during your continuing experiments ?

Kind Regards , Trevor

Hi Trevor,
The engine was designed to rev to around 10k max, and has never significantly gone over this. However after hearing Mark riding round Cadwell recently I fully beleive I should step up to the challenge!
I had hoped to stay competitive with a strongly torquey motor, mine is very similar in characteristics to the Pete Tibbitts engine (not as quick though), and consequently we alway ran very similar sprockets at all circuits. I don't think I build a bottom end as well as Pete though! and the gearbox has been a nightmare from time to time, but the barrel has been a proverbial brick and never ever seized... holed a few pistons mind you!
My fave moment ever, was giving Pete a hard time at Cadwell Woodlands circuit, and I even managed to get past at the hairpin! I bowed to the better man by the end of the race though!
(see picture in the "Old Bike Mart")
Cheers,
Alan

You both talk of "Max Revs" as if you are talking of `Maximum Full-Load Revs´ and there must be a Maximum NO-LOAD revs which you never mention -- which can be seen on the dyno as such. Or don´t you look for it during dyno test? Also it (the max No Load speed) shows on the rev counter when slipping the clutch whilst pulling away -- or do you use a different language from me for that condition?

Hi John,
Yes, I only consider max "full load revs" as being of any interest, but when I used a rev counter I did get 10k rpm on the clock on the grid just as I pull away, so that is as near as I can tell you about max "No Load" rpm!
I do know others are getting considerably more, because my ear tells me!
Cheers,
Alan

Hi Folks

With all the mention of engine revs, just wondering what type of rev counter is most suitable for a racing Bantam, traditional chronometric or electronic?

Hi Alan!
You mentioned your power curve is similar Peter´s and I have Peter´s (125) which he gave me permission to use. It differs enormously from the 175 curves Mike
showed titled "Ian´s/Jimmy´s" where Peter´s springs sharply upwards and exceeds their max full-load curve by 700rpm: i.e. I/J´s (175) at 9,500rpm(22 bhp) ...
Whilst the I/J´s curves are mole-hill shapes Peter´s is like a jagged ice splinter pointing heavenward!!
Certainly in my whole snatch I have not seen such a power curve: Peter attributes this `High Torque´ and rapidly increasing power curve to the reed valve.
Since -- in my Bantam racing time -- reed valves were for me akin to pre-war Kenyan Kijkuyus knowing about television in 1934 I am certainly learning something -- in 2011 -- from all this ....!??!

Last point concerning high compression-ratio and tetraethyl-lead doping is that the incoming colder fuel cools the incoming air-charge and the combustion period is longer (major heat release being well after TDC) with lower peak combution pressures -- hence 16:1 and above are normal CRs with tetraethyl fuels....

Aaahhh(yawn)mmmen!

PS -- I just deleted Peter´s engine power because it happens to be Nick B´s now and I didn´t have Nick´s permisssion to say....
JayBee.

John - think you are confused? Details from me entitled "Jimmys 175" (not Ian) was the bike i borrowed of Jimmy in 2008 when i didn't have a complete usable bant of my own. The bike(and new version engine) now belongs to IOM exponent Tom Snow and was campaigned at the last Cadwell - interestingly lapping at the same times i did 3 years ago(on the old motor).

Trevor are you getting back to these keen 175 racers...?

Hello Mike ,
Vitriol and opprobrium tend to suffocate enthusiasm, i am not sure my offerings are now worth the effort !

More than sad, but there you are !


Trevor

Whoops!
Mike! you published two dreadfully scrawly power curves -- in red & green-- on the same `www.dynomax.com´ form -- with written up in the corner, "Jimmy´s 175 motor" that had underneath, "Mick Scutt Pipe" -- which I used in my "Heavier Flywheel Treatise" as "Jimmy & Ian´s Pipes" -- ooh-ohhh, ooopsy -- whoopsie!!.
Must get new specs because unfortunately I saw the word "Ian`s" when it was
`Mick´...

You are correct -- my error! But I must excuse myself by saying that I found those
curves so uncontrolled wandering all over the place as to effect my eyesight permanently...

However, this does give me the opportunity to be critical -- what excuse did www.dynomax.com give for such an ... unprofessional presentation??

Just to keep on the nice side of you Mike I sent my tenner contribution to a trophy
by pidgeon post this morning so if you don´t get it you´ll know a Postman Pete
is having a pint on the Bantam Boys.

Cheers!
JayBee.